selftests/bpf: fix missing BPF object files

selftests/bpf: fix missing BPF object files

After commit afef88e65554 ("selftests/bpf: Store BPF object files with
.bpf.o extension"), we should use *.bpf.o instead of *.o.

In addition, use the BPF_FILE variable to save the BPF object file name,
which can be better identified and modified.

Fixes: afef88e65554 ("selftests/bpf: Store BPF object files with .bpf.o extension")
Signed-off-by: Wang Yufen <wangyufen@huawei.com>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/1666235134-562-1-git-send-email-wangyufen@huawei.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>